Why do Christians sing when they are together?
"...because in singing together, it is possible for them to speak and pray the same Word at the same time; inotherwords, because they can unite in the Word."
- Bonhoeffer, Life Together
"I will sing a new song to you, O God; on the ten-stringed lyre I will make music to you..."
Ps. 144:9.
"Speak to one another with psalms, hymns and spiritual songs. Sing and make music in your heart to the Lord." Ephesians 5:19
FYI - to sing a new song to the Lord, it must first be sung in my heart.